There are plenty of theories about what dark matter is and how it might be gravitationally affecting the universe. However, proving those theories out is hard since it hardly ever interacts with anything, especially on “small” scales like galaxies. So when a research team claims to have found evidence for dark matter in our own galaxy, it’s worth taking a look at how. A new paper from Dr. Surkanya Chakrabati and her lab at the University of Alabama at Huntsville (UAH) does just that. They found evidence for a dark matter “sub-halo” in the galactic neighborhood, by looking at signals from binary pulsars.
A sub-halo is a clumping of dark matter that is brought together inside of a larger “halo” that is thought to form the core of galaxies. Since dark matter primarily interacts through gravity, going theory suggests that it should attract “baryonic” (i.e. normal) matter when it clumps together. This clumping is thought to the scaffolding that galaxies are built on.
Sub-halos are even denser groupings of dark matter that coalesce because of their gravitational attraction. Since they are relatively small compared to the big dark matter halos they are contained in, they can be difficult to detect. To do so, cosmologists would have to find a gravitational signal that deviates from what would be expected given the normal matter surrounding the sub-halo. So far, no one has been able to isolate that kind of signal, despite looking throughout our galactic neighborhood.
Fraser discusses how much dark matter there is in the solar system.
Enter binary pulsars – these star pairs contain at least one pulsar, a type of neutron star which emits a large amount of energy on a regular cycle (hence their name). These bursts can be measured so accurately they rival atomic clocks in terms of regularity. The researchers had a theory that they could use deviations in that expected cycle to detect the gravitational effects of a dark matter sub-halo, so they began looking at binary pairs in the galaxy to see if they could find any hint of it.
Overall they looked at 27 binary pulsars, and in particular were looking for gravitational changes between two pairs, to increase the chance there was indeed a structure causing the deviation. They found two, called PSR J1640+2224 and PSR J1713+0747, that had the kind of significant correlated gravitational change they were looking for.
To isolate that gravitational change, the researchers had to eliminate other forms of gravitational acceleration that could be caused by things other than dark matter. One is “gravitational radiation”, the acceleration caused when the system gives off gravitational waves, and predicted by the theory of general relativity. Another is the Shklovskii Effect, which is an artifact caused by a binary system moving across our line of sight. Thankfully, both of these effects are well understood and can easily be removed from the calculation of the gravitational influence on the binary system.
Fraser discusses our ongoing search for dark matter and some of the different ways we hope to detect it.
Some of that gravitational influence can still come from baryonic matter, but in the case of these two binaries there appeared to be a substantial component that couldn’t be explained that way. In fact, the statistics of that additional component were so compelling its hard to argue that it was caused by anything other than an unseen gravitational mass.
Defining that mass was the next step. The researchers pinpointed it at about 2,340 light years away, and determined its mass to be around 2.45 x 107 solar masses. An equivalent amount of baryonic matter causing that gravitational change would be 100 times what is observable in that part of the galaxy.
This research represents the first time a dark matter sub-halo has been detected in the general galactic neighborhood, after having been predicted by theory for years. It also offers a technique by which other researchers could do the same with other sets of binary pulsars. Though rare, astronomers are continually collecting new data on them constantly, giving cosmologists even more data to analyze. Likely this won’t be the last time we’ll hear of this technique being used to find dark matter sub-halos – there are plenty more places to search for them, and likely many more to discover.
Learn More:
UAH – UAH researchers use pulsar accelerations to detect a dark matter sub-halo in the Milky Way for the first time
S. Chakrabarti et al – Constraints on a dark matter sub-halo near the Sun from pulsar timing
UT – Tying Theory To Practice When Searching For Dark Energy
UT – Astronomers Search for Dark Matter Using Far Away Galaxies